During a clinical high dose rate (HDR) brachytherapy procedure catheters are inserted into a target region within a person, wherein through the inserted catheters radiation sources are introduced into the target region in accordance with a treatment plan, which defines dwell times and dwell locations, for treating the target region. The treatment plan is determined in advance based on, inter alia, three-dimensional poses and shapes of the inserted catheters, wherein for determining the three-dimensional poses and shapes of the catheters within the person a user introduces sequentially a guidewire into the catheters, while the position of the tip of the guidewire within the respective catheter is electromagnetically tracked. Thus, the treatment plan, particularly the dwell positions, is determined based on, inter alia, the electromagnetically tracked positions of the tip of guidewire. Determining the treatment plan based on the positions of the tip of the guidewire within the respective catheter and also delivering the treatment based on, inter alia, the electromagnetically tracked positions of the tip of the catheter can be inaccurate, thereby reducing the quality of the brachytherapy.